Recently, as being accompanied by the developments of portable electronic devices such as cellular phones and notebook-size personal computers, or as being accompanied by electric automobiles being put into practical use, and the like, small-sized, lightweight and high-capacity secondary batteries have been required. At present, as for high-capacity secondary batteries meeting these demands, non-aqueous secondary batteries have been commercialized, non-aqueous secondary batteries in which lithium cobaltate (e.g., LiCoO2) and the carbon-system materials are used as the positive-electrode material and negative-electrode material, respectively. Since such a non-aqueous secondary battery exhibits a high energy density, and since it is possible to intend to make it downsize and lightweight, its employment as a power source has been attracting attention in a wide variety of fields. However, since LiCoO2 is produced with use of Co, one of rare metals, as the raw material, it has been expected that its scarcity as the resource would grow worse from now on. In addition, since Co is expensive, and since its price fluctuates greatly, it has been desired to develop positive-electrode materials that are inexpensive as well as whose supply is stable.
Hence, it has been regarded promising to employ lithium-manganese-oxide-system composite oxides whose constituent elements are inexpensive in terms of the prices as well as which include stably-supplied manganese (Mn) in their essential compositions. Orthorhombic-system o-LiMnO2 is a compound that has charging/discharging profiles in 4-V region and 3-V region, respectively. The charging/discharging behavior of lithium-ion secondary battery at room temperature, lithium-ion secondary battery in which the orthorhombic-system o-LiMnO2 is used as the positive-electrode active material, is introduced in Non-patent Literature No. 1, and the like. According to this literature, the lithium-ion secondary battery in which the o-LiMnO2 is used exhibits such a behavior that it has a very small capacity (e.g., at around 50 mAh/g) in the beginning of charging/discharging and the capacity reaches a maximum capacity (e.g., 100 mAh/g or more) by repeating the charging and discharging up to a few dozens of cycles more or less. However, no such batteries whose capacities are low up until arriving at higher cycles have been desired practically.
Moreover, although it has been known that the electrochemical characteristics of LiMnO2 depends on the synthesis methods, the synthesis is difficult compared with those of the other composite oxides. For example, in Patent Literature No. 1, a non-aqueous-system secondary battery exhibiting a high capacity relatively, non-aqueous-system secondary battery in which LiMnO2 is used as the positive-electrode active material, is disclosed. Here, LiMnO2 is synthesized by heat-treating a pellet, which comprises an MnCO3 powder and an Li2CO3 powder, at 1,000° C. for 12 hours and then cooling it rapidly. Moreover, although another synthesis process, in which reactions are caused at low temperatures relatively with use of hydrothermal treatment, has also been known, no sufficient capacity is obtainable even when the thus obtained LiMnO2 is employed in lithium-ion secondary batteries. Although it is useful industrially if it is possible to synthesize LiMnO2, which brings about higher capacity, at a lower temperature and for a shorter period of time, it is the actual situation that no such a synthesis process has been established yet.
Moreover, in positive-electrode active materials for lithium-ion secondary battery, many of them are those in which the diffusion rates of lithium ion within their composite oxides are small relatively. As one of means for charging and discharging such composite oxides at a faster rate by letting lithium ions come in and go from out of them in a short period of time, it is possible to think of making the composite oxides' particle diameters smaller. Consequently, it has been needed to develop a simple and easy synthesis process for fine particles. For example, in Patent Literature No. 2, a process for synthesizing nano-order oxide particles is disclosed. In Example No. 3 of Patent Literature No. 2, MnO2 including tetravalent Mn, and Li2O2 are added to and are then mixed with a mixture, in which LiOH2H2O and LiNO3 are mixed in a molar ratio of 1:1, thereby synthesizing lithium manganate being expressed by LiMn2O4 whose manganese has an average oxidation number that is equal to a valence number of 3.5, by turning the mixture into molten salt after letting the mixture go through a drying step.